An example of a particle beam microscope is a scanning electron microscope, in which a focused electron beam is scanned over an object to be examined and secondary electrons or backscattered electrons, generated by the incident electron beam at the object, are detected in a manner dependent on the deflection of the focused particle beam in order to generate an electron-microscopic image of the object.
The particle beam is generated by a particle source and accelerated, it possibly passes through a condenser lens and it is focused at the object by an objective lens. In order to obtain a high spatial resolution of the particle beam microscope, the particle beam is focused to the best possible extent at the object, i.e. a region illuminated by the focused particle beam at the surface of the object (“beam spot”) should be as small as possible. In practice, this is achieved by virtue of a user setting the focusing of the particle beam by hand by actuating an actuating element, such as a focus adjustment wheel, and the controller of the particle beam microscope changing the excitation of the objective lens in a manner dependent on the actuation of the actuating element. During this adjustment process, the particle beam is scanned continuously over the object in order to record images. The user can assess the quality of the current images and, in a manner dependent thereon, actuate the actuating element until he is satisfied with the quality of the images or he can no longer improve the quality thereof. Then, within the meaning of the assessment of the quality of the images carried out by the user, the particle beam is focused to the best possible extent. However, this process is also time-consuming.
The kinetic energy with which the particles are incident on the object is given by the difference between the electric potential of the particle source and the electric potential of the object. Electric fields are present between the objective lens and the object in some particle beam microscopes in order to provide a lens effect for focusing the particle beam and/or in order to reduce the kinetic energy of the particles prior to incidence on the object. In some cases, it is desirable to change the electric potential of the object in order to bring about a modified observation situation, which generates a change in contrast of the image being created. A change in contrast is brought about by virtue of electric fields at the object being modified by the modified potential of the object, which has an influence on the trajectories of backscattered electrons and, in particular, secondary electrons and on the acceleration thereof toward a detector. As result of this, it is possible to make certain structures of the object visible in the image, which structures are not visible in a different observation situation. With a change in the electric potential of the object, there is a change in the kinetic energy with which the particles are incident on the object, and there is also a change in the electric fields between object and objective lens. Both have an influence on the focusing of the particle beam at the object, and so the user once again sets the focusing of the beam after a change in the electric potential of the object.